For purposes of the present specification, "mechanical joint assembly" shall denote an assembly which allows two members to be mechanically joined while allowing rotation of one member with respect to the other about at least one rotation axis. "Universal joint" shall denote a mechanical joint assembly providing rotation about at least two substantially orthogonal axes. Such a joint may be useful in applications in which two members must be mechanically joined but must be allowed to assume an arbitrary relative angle.
In some applications it is desirable for the motion of the moving members about the universal joint to be restricted by damping and/or braking. For example, such a universal joint is useful in situations wherein a first joined member is a boom with a second joined member suspended therefrom being some load carrying means, wherein motion of the load carrying means must be restricted, particularly when not loaded. One particular application of a braked universal joint is suspension of a grappler from a boom of a logging skidder. Several previous designs for a braked universal joint used in this way (also referred to as a swivel link) are described in U.S. Pat. Nos. 4,335,914; 4,417,759; 4,572,567; 4,573,728; 4,679,839; 4,715,641; 4,717,191; 4,723,639; 4,810,020; 5,096,247; 5,110,169; 5,451,087; and 5,601,161, each of said patents being incorporated by reference as if fully set forth herein. A much improved design for a swivel link incorporating frusto-conical surfaces for braking and load-bearing is disclosed in U.S. Pat. Nos. 5,713,688 and 5,779,383 issued to the applicant of the present application, both of said patents being incorporated by reference as if fully set forth herein. Another improved design for a swivel link employing rotational position-dependent braking torque is described in co-pending application Ser. No. 09/087,719, now U.S. Pat. No. 6,119,824 filed in the name of the applicant of the present application, said application being incorporated by reference as if fully set forth herein. The design and construction of swivel link assemblies, many drawbacks of previous swivel link designs, and improvements resulting from the use of frusto-conical braking and load-bearing surfaces are fully disclosed in these patents and application, and need not be reiterated herein.
A primary figure-of-merit for a swivel link is the number of hours of use in the field before replacement of the friction members of the joint (friction discs in older designs, friction cones in the frusto-conical design). Anything that reduces wear of the braking/friction members (and therefore reduces concomitant down time, maintenance time, and maintenance costs) is highly desirable.
During use of a swivel link, and many other braked mechanical joints, it is often the case that slow motions about the joint are insignificant and can be tolerated, while faster motions are undesirable and must be reduced or prevented (for example, to prevent injury and/or equipment damage). However, previous joints provide a constant braking torque relatively independent of the speed of motion. Reduction of the braking torque for slow rotations would reduce unnecessary wear on the braking/friction members, "saving" the braking/friction members for suppression of faster rotations.
In addition, earlier braked joint assemblies have relied on friction for generating braking torque. Mating surfaces are thrust together to generate this friction, and these surfaces must necessarily wear during use of the joint. Obviously, these surfaces could not be lubricated to reduce wear. A joint in which braking torque was not generated by friction would not be subject to such wear, could be thoroughly lubricated to reduce wear of moving surfaces, and would therefore have a longer operational lifetime before requiring maintenance or replacement.
It is therefore desirable to provide a damped mechanical joint assembly in which a relatively smaller damping torque is applied during slower motions of the joint, while a relatively larger damping torque is applied during faster motions of the joint. It is be desirable to provide a damped mechanical joint assembly in which the damping torque is generated without the use of friction members.